Clarification of bleach liquors



Fatenteel Dec. 7, rue.

U T 1? T d JAMES DOUGLAS MACMAHON, 0F NIAGARA FALI4S, NEW YORK, ASSIGNGRTO THE .MATHIESON ALKALI WORKS, OF NEW YORK. N. 521, A CORPORATION OFVIRGINIA.

CLIFICATIUN 0F BLEACH LIQUORS.

No Drawing.

This invention relates'to the clarification of calcium hypochloriteliquors such as are used in the pulp and paper industries for bleaching.Such bleach liquors are com monly made either by dissolving bleachingpowder in water or by chlorinating suspensions of lime in water.Commercial bleaching powder always contains some excess free lime, and,when the bleaching powder is dissolved, this free lime, excepting thesmall amount dissolved, is suspended in the solutionwWhen the bleachliquor is prepared by chlorinating a suspension of lime, an excess oflime is used to produce a stable liquor and to facilitate control of thechlorinating operation. The lime employed may also contain otherassociated insoluble mineral matter, and where bleaching powder is usedinsoluble impurities such as calcium carbonate may also be present, Theresult is that the bleach liquors, as commonly prepared, always containsome suspended lime 1n varymg degrees of SlllJ-CllVlSlOIl and frequentlycontain other insoluble impurities.

ln the pulp and paper industry it has been found that such suspendedmineral matter, and particularly free. undissolved lime, is seriouslydetrimental to the eliective use of the bleach liquors. To overcome thisdifficulty it has been customary to subject the bleach liquor, aftersolution of the bleaching powder or chlorination of the lime suspension,to long periods of settling, followed by decantation of the supernatantliquor lromthe precipitated residue. Not only have such methods involvedlarge storage capacity in the bleach liquor plant but extreme precautionis necessary in the decantation operation, and, no matter how carefullythe .decantation is made, or how long the period of settling, a certainper centage ol" finely divided lime always re mains in suspension in thebleach liquor. in some cases even filtration ot the bleach liquor hasbeen resorted to for effecting separation of finely divided suspendedlime. A part of the lime present is in a. very fine state ofsub-division and it is particularly this very finely divided materialwhich will not settle out readily that causes trouble, both in removaland in subsequentuse oi the bleach liquor it it is not removed.According the present invention, the

, bleach liquor is clarified by chlorinating the finely dividedsuspended eras Application filed March w, 1925. Serial No. 14,576.

after an initial separation of the coarse suspended material withoutcarrying the chlorination beyond neutrality. A clear stable bleachliquor, free from undissolved tree lime, can thus be produced, while thelong periods of settling and large storage capacity and the extremeprecaution in decantation or the filtration hitherto required are.eliminated. The method offthe invention is applicable to bleach liquorsprepared either by solution of bleaching powder or by chlor ination oflime suspensions, but it is particularly eiiective in the clarificationof chlorinated suspensions containing an ex cess of lime. p

In carrying out the invention, the bleach liquor may be initiallyprepared in the customary Way and in the concentrationscommonly used.After initial preparation of the bleach liquor, for example, either bydissolving bleaching powder in Water or by chlorinating an aqueoussuspension of lime, e. g. milk of lime, it may be settled for arelatively short period and the coarse suspended matter so precipitatedthen separated from the liquor. While the very finely divided susupendedlime requires a very long if not an indefinite period to settle, thecoarse particles of suspended material settle comparatively rapidly and,only a com-.'

paratively brief period is required,.for example, the period of settlingmay be reduced to one-half or less of that now commonly employed.Following the separation at coarse suspended particles the partiallyclarified liquor is removed, as by decantetion. The extreme precautionhitherto required in such operations may also be relaxed here because ofthe subsequent chlorination operation. The partially clarified bleachliquor then contains in suspension the finely divided lime particleswhich will not settle out readily.

The bleach liquor, which is then turbid is then chlorinated until theremaining lime in suspension goes into solution. The

very finely divided lime ditficult if not impossible to removecompletely by the methods hitherto employed, is thus dissolved by thechlorination operation without the necessity of prolonged settling andcareful decanta due to free undissolved lime in suspension, m

effect a complete separation of the coarse material in the initialclarification is also corrected by the chlorination operation. Thechlorination is not carried beyond neutrality. A clarified stable bleachliquor free from undissolved free lime is thus produced.

It will thus be seen that the present invention provides an improvedmethod for eliminating free undissolved lime from bleach liquor, andconsequently of overcoming the difficulties encountered in theapplication of such bleach liquors in the pulp and paper industry. Theinvention effects a substantial saving in time and also reduces thestorage capacity required in any given instance. Conversely, theinvention substantially increases the capacity of any existing bleachliquor plant. The time required .and the storage capacity required maybe reduced as much as 50% or more. The invention also enables theproduction of bleach liquor free from suspended free lime to a degreehitherto approached only by. filtration methods but without requiringthe carrying out of such operations. The process of the invention,moreover, may be carried out in existing bleach liquor plants withoutsubstantial modifica tion. It will be apparent that the invention is ofmore or less general application to the clarification of bleach liquorswhere the elimination of free undissolved lime is de sirable.

I claim: I

1. An improvedmethod of clarifying calcium hypochlorite bleach liquorscontaining suspended mineral matter including lime, which comprisesseparating from the bleach liquor coarse-particles of suspended mineralmatter and subsequently chlorinating the separated liquor until theremaining finely divided suspended lime dissolves, without carrying thechlorinat on beyond neutrality.

2. An improved method of preparing bleach liquors, which compriseschlorinating an aqueous suspension containing an excess of lime,separating from the chlorinated liquor coarse particles of suspendedmineral matter, and subsequently further chlorinating the separatedliquor until the remaining finely divided suspended lime dissolves,Without carrying the chlorination beyond neutrality.

In testimony whereof I aflix my signature.

JAMES DOUGLAS MAQMAHON.

